how to update a value of a dict withing a loop into a list of dict

Question

As I am stuck, I need some help. I want to update a value of a dict in a list of dict (ideally, I don't want to create another list)

Under an Ansible playbook, I have a variable declared:

a:
  - id: 'test1id'
    name: 'test1name'
    description: ''
  - id: 'test2id'
    name: 'test2name'
    description: 'init'
  - id: 'test3id'
    name: 'test3name'
    description: 'init3'

I have a task that loop into "a" should update the description.

- name: Update description in a
  when: a is defined
  set_fact:
    a: "{{ item | combine({'description': new_description}) }}"
  vars:
    new_description: "{{ item.description + "test" + item.id }}"
  loop: a

Expected result:

a:
  - id: 'test1id'
    name: 'test1name'
    description: 'test test1id'
  - id: 'test2id'
    name: 'test2name'
    description: 'init test test2id'
  - id: 'test3id'
    name: 'test3name'
    description: 'init3 test test3id'

Actual result:

a:
  - id: 'test3id'
    name: 'test3name'
    description: 'init3 test test3id'

Looks like I'm missing something, I just have the last item. I also tried other format, but that never give me the expected result.

Answer 1

What you've done is right.
There were some errors, but I think it was caused by a typo, like loop: "{{ a }}" and spaces around the word test here new_description: '{{ item.description + " test " + item.id }}'.

If you run it in debug mode:

ok: [localhost] => (item={'id': 'test1id', 'name': 'test1name', 'description': ''}) => {
    "ansible_facts": {
        "a": {
            "description": " test test1id",
            "id": "test1id",
            "name": "test1name"
        }
    },
    "ansible_loop_var": "item",
    "changed": false,
    "item": {
        "description": "",
        "id": "test1id",
        "name": "test1name"
    }
}
ok: [localhost] => (item={'id': 'test2id', 'name': 'test2name', 'description': 'init'}) => {
    "ansible_facts": {
        "a": {
            "description": "init test test2id",
            "id": "test2id",
            "name": "test2name"
        }
    },
    "ansible_loop_var": "item",
    "changed": false,
    "item": {
        "description": "init",
        "id": "test2id",
        "name": "test2name"
    }
}
ok: [localhost] => (item={'id': 'test3id', 'name': 'test3name', 'description': 'init3'}) => {
    "ansible_facts": {
        "a": {
            "description": "init3 test test3id",
            "id": "test3id",
            "name": "test3name"
        }
    },
    "ansible_loop_var": "item",
    "changed": false,
    "item": {
        "description": "init3",
        "id": "test3id",
        "name": "test3name"
    }
}

You just need to read the values back from ansible_facts you set.
The result should be registered temporarily and then convert the results to the original format with the updated values.

- hosts: localhost
  gather_facts: no
  vars:
    a:
    - id: 'test1id'
      name: 'test1name'
      description: ''
    - id: 'test2id'
      name: 'test2name'
      description: 'init'
    - id: 'test3id'
      name: 'test3name'
      description: 'init3'
  tasks:
  - name: Update description in a
    when: a is defined
    set_fact:
      a: "{{ item | combine({'description': new_description}) }}"
    vars:
      new_description: '{{ item.description + " test " + item.id }}'
    loop: "{{ a }}"
    register: temporary_a

  - name: Results
    set_fact:
      a: "{{ temporary_a.results | map(attribute='ansible_facts.a') | list }}" 

  - debug:
      var: a

Results:

ok: [localhost] => {
    "a": [
        {
            "description": " test test1id",
            "id": "test1id",
            "name": "test1name"
        },
        {
            "description": "init test test2id",
            "id": "test2id",
            "name": "test2name"
        },
        {
            "description": "init3 test test3id",
            "id": "test3id",
            "name": "test3name"
        }
    ]
}